Method for preservation under pressure

ABSTRACT

Method for substantially decreasing the viability of microorganism and deactivating enzymes in a contaminated substance by exerting a high pressure the substance. The substance is conducted in a steady flow through and open narrow tube while the pressure difference between the entrance and the exit of the tube is maintained at 100 Mpa or more. The temperature rise of the product while passing through the tube can be confined to less than 5° C. The method provides a continuous ultra high pressure preservation process suitable for processing foods.

This application is the national phase of international applicationPCT/EP97/02711 filed May 15, 1997 which designated the U.S.

The present invention is concerned with a method for preservation,particularly ultra high pressure preservation. The method is used foroperation in a continuous mode and is particularly suitable in the foodindustry.

STATE OF THE ART

Industrially prepared food usually has to be subjected to a preservationtreatment in order to prevent spoilage during subsequent storage.Ultra-high pressure (UHP) preservation is a preservation method whichonly relatively recently has been developed for industrial application,although the lethal effect of ultra-high pressure on micro-organisms hasbeen discovered already in the previous century by B. H. Hite. A reviewof the state of the art can be found in New Methods of Food Preservation(1995, ed. G. W. Gould). UHP preservation is the subject of manypatents: e.g. U.S. Pat. No. 4,873,094, U.S. Pat. No. 5,228,394. NL 102914 describes conducting a spread-like product through a narrow tubeunder an initial pressure of 40 atmospheres with a beneficial effect onthe consistency of the product. This pressure however, is not highenough to have a significant effect on the viability of micro-organismsin the product.

Substances treated in a homogenizer are exposed also to a very highpressure, but during a very short time (several milliseconds). In such adevice the shear forces exerted on the substance during the pressuredrop are enormous and often damage the product structure. Moreover theenergy needed for passing the product through the homogenizing clearancedissipates quickly in a small volume of the shearing device resulting ina local, unacceptably high temperature rise. Usually this rise isapproximately 5° C. per 20 MPa of pressure drop, the rise also dependingon the thermal capacity and heat conductivity of the product.

A major disadvantage of known UHP preservation techniques is that UHPpreservation is applied only batch-wise. Since 20 most food processingis operated in a continuous mode, an UHP preservation method which couldbe operated as a continuous process would fulfill a need. Only WO95/22912 describes UHP equipment with which a semi-continuous processcan be carried out. Present equipment for UHP processing is complicatedand so expensive that it impedes an economic use and consequently thegeneral employment of UHP preservation.

STATEMENT OF INVENTION

We have found an unexpectedly feasible combination of two seeminglycontroversial conditions: the one condition being maintenance of a highkinematic pressure in a tube which is relatively narrow and open at theexit end, which kinematic pressure in at least a part of the tube issufficiently high that a microbiologically contaminated fluid during itsflowing through the tube gets decontaminated, the other condition beingthe realisation of a flow which is high enough to make the processeconomically feasible.

The invention therefore provides a method for decreasing the viabilityof micro-organisms and/or the activity of enzymes in a contaminatedsubstance by exerting a high pressure to the substance, characterized inthat the substance is conducted in a steady flow through a tube, whilethe pressure difference between the entrance end and the exit end of thetube is maintained at 100 MPa or more. The present method allows a fullycontinuous UHP preservation process.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows a schematic view of the equipment with which the inventioncan be carried out. c is a tube with a length L and an inner diameter d.a is a storage container connected to the entrance of the tube via apressure unit b. At d the open orifice of the tube is situated.

DETAILS OF THE INVENTION

The invention essentially is carried out by feeding the substance fromthe storage container a to the entrance of the tube via a pressure unitb and passing it through the tube to the exit at the right hand side.

The invention can be applied on all types of fluid substances which needa decontamination treatment, provided they have a consistency whichallows a sufficiently quick passage through the necessarily small tubesemployed with the invention. Such substances comprise pharmaceuticalsubstances, clinical liquids, and particularly food products such asspreads, mayonnaise, dressings, milk, tea and even heat sensitiveproducts as ice-cream and soft cheese. The invention is particularlysuitable for substances which tolerate only gentle treatments. Thesubstance may be a final food product or an ingredient (or a mixture ofingredients) used for the preparation of a 5 food product, includingeven such nature originating substances as herbs, provided they can beincorporated in a fluid carrier substance which can be pumped throughthe narrow tube.

In order to maintain a pressure of at least 100 MPa between the entranceand the exit of the tube, a proper balance should be found between onthe one side the diameter and length of the tube and on the other sidethe given viscosity and the desired flow of the product to be treated.The minimum volume V of the tube results from the formula

    V=t*f,

where t is the minimum residence time for effective decontamination andf is the desired flow. The residence time can be adjusted withoutchanging the narrow tube dimensions by inserting a chamber at theupstream end of the narrow tube, between the exit of the pressure deviceand the entrance of the narrow tube. With such chamber the ultra highpressure volume is increased and consequently the residence time of thefluid. Because of its resistance against high pressures, such chamberpreferably is a tube too, which diameter is greater than the narrow tubediamter so that pressure drop and flow are not substantially influencedby the presence of the chamber. Preferably such chamber has a diameterwhich is at least 5 times greater than the narrow tube diameter. Thefollowing description of a tube is not applicable to this residencechamber, but rather to the attached narrow tube. Unless it is indicatedotherwise, the term tube is used for the narrow tube.

In the context of the present description a tube is considered to be around vessel with two openings at both ends of the vessel where thelength of the vessel is at least ten times the width of the vessel.Generally, the ratio of the length and the average diameter of a tubesuitable for the invention is at least 1000, preferably at least 10,000.Generally this means a diameter of only several millimeters and a lengthof at least several meters. The optimum dimensions can be easily foundby some calculation and experimentation. Good results can be obtainedwith a tube having a length of only 200 m and an internal diameter of 10mm. It is much surprising that food products which often have a ratherviscous consistency can be pressed through such tube at a flow ratesufficient for economic processing. With said open tubes an output perhour of about 50 liter product having an oily viscosity can be realizedby exerting a pressure of 1000 MPa. The high flows needed in practiceare realized by combining into bundles large numbers of parallel tubes.See also Table I for examples of suitable tube dimensions in relation togiven substance viscosity and exerted pressure.

Pressure building in an open tube was believed to be possible only withextremely long tubes. However, an unexpectedly favourable pressuredependent viscosity behaviour is observed.

The pressure within the tube should be at least 100 MPa, but pressuresof at least 300 MPa are preferred. Generally higher pressures allowshorter decontamination times.

The ultra high pressures needed for working the invention can bewithstood best by tubes with relatively narrow diameters: 10 mm or lessis preferred. Special reinforcement is not necessary. The presentpreservation device does not need the very thick walls of prior artequipment.

                  TABLE I                                                         ______________________________________                                              P      L        d             Visc. Flow                                Fluid MPa    m        m     L/d     Pa · s                                                                     l/h                                 ______________________________________                                        1     751    100      0.0010                                                                              100000  0.001 50                                  2     566    100      0.0010                                                                              100000  0.01  50                                  3     559    100      0.0015                                                                              66667   0.05  50                                  4     539    100      0.0018                                                                              55556   0.1   50                                  5     377    100      0.0035                                                                              28571   1     50                                  6     437    100      0.0060                                                                              16667   10    50                                  7     566    100      0.0100                                                                              10000   100   50                                  8     546    100      0.0012                                                                              83333   0.1   10                                  9     707    100      0.0020                                                                              50000   0.1   100                                 10    442    100      0.0040                                                                              25000   0.1   1000                                11    699    1000     0.0030                                                                              333333  0.1   50                                  12    566     10      0.0010                                                                              10000   0.1   50                                  13    354     10      0.0020                                                                               5000   1     50                                  ______________________________________                                         Applicable to fluids having a density of about 1000 kg/m.sup.3 and a heat     capacity of 4.2 J/g · K                                              p: pressure drop in Megapascal                                                d: average diameter of tube in meters                                         Flow: flow rate in liters per hour                                            L: length of tube in meters                                                   Visc: viscosity in Pascal seconds                                        

The tube may be placed in any position, but preferably a compact formsuch as a coil is chosen. Tubes having a circular intersection are mostadvantageous in resisting high pressures, but other forms ofintersections are not excluded.

Glass and stainless steel, substances which are compatible with food,are preferred tube materials.

For the pressure device or unit a choice can be made from the devicesfound on the market which are meant for pumping fluids under ultra highpressures.

In order that the exerted pressure has a sufficient effect on themicro-organisms, the residence time of the fluid in the tube should beat least 1 second. Generally, longer residence times are needed when thepressure is lower than 350 MPa. Preferably the residence time is atleast 2 minutes, more preferably at least 5 minutes and still morepreferably at least 10 minutes.

It is difficult to give general rules since the flow behaviour of thesubstance processed under UHP conditions generally can not be predicted.Given a particular substance, some experimentation will easily providethe proper combination of tube dimensions and pressure.

The present device operates with a permanently open orifice at the endof the tube. The effect is a pressure gradient along the whole length ofthe tube. Consequently the pressure in the tube is higher in upstreamparts than in downstream parts of the tube, with the effect thatdecontamination takes place predominantly in the upstream part of thetube.

High pressure energy is dissipated evenly over the whole length of thetube.

Within the tube the shear forces are relatively small. Moreover both therelatively large external surface of the tube in relation to the volumeof the tube and the relatively thin wall of the tube allow an easycontrol of the temperature of the tube's content if necessary with thehelp of additional cooling. The temperature rise of the processedsubstance during tube passage can be confined to less than 10° C.,preferably less than 5° C. This fits into modern concepts to avoid asmuch as possible unnecessary heating of industrially prepared food.

Operating the process at a temperature different from ambienttemperature may be advantageous. When the temperature is lowered, theviscosity will increase which makes it possible to maintain the pressureat the desired level even when the fluid to be treated is notsufficiently viscous at ambient temperature.

A temperature increase will cause a lowered viscosity and anadvantageous increase of the flow will result. Such increase will meetthe obvious limitation that the substance to be treated needs a minimumresidence time in the tube.

The present invention gives a method which allows the decontamination offood products where the use of preserving ingredients, a low pH or theuse of heating is undesirable.

Nevertheless the present UHP method may be used in combination with oneor more other preservation methods. When combining methods, often muchless severe over-all conditions will suffice for attaining the requireddecontamination degree.

A particularly effective combination is the application of lethalpulsating electrical or magnetic fields to the substance when it passesthrough the high pressure tube.

The process of the invention inactivates vegetative cells. For theinactivation of microbial spores generally a higher pressure and/or alonger exposure time should be applied. Affected micro-organisms includebacteria as well as moulds and yeasts, but also viruses. Although fullsterilization of the product in principle is possible, often a lesserdegree of decontamination suffices, so that less severe processconditions can be applied.

UHP preservation has the additional advantage that also enzymes arefully or partially deactivated.

In the context of this specification with a substantial decrease inviability is meant a reduction in the viable microorganisms count with afactor 1000 or higher. This is often expressed as logcycle reduction(log (N0/Nt)) which should be 3 or higher. Nt is the count after theprocess and N0 before the process.

The present method distinguishes itself from prior art methods by itssurprising simplicity which not only contributes to economy but also toprocess reliability.

The invention is further illustrated by the following example:

EXAMPLE 1

In 1000 ml of glycerol 1000 cells per ml of the yeast Saccharomycescerevisiae have been dispersed. The dispersion in which a naturalcontamination condition was emulated was conducted through a tube with alength of 25 m and a diameter of 1 mm with a pressure of 300 MPa at theentrance of the tube. The residence time in the tube was 60 seconds andthe temperature was ambient temperature, 21° C. The substance collectedat the end of the tube was assayed on contamination, but no detectableamount of yeast cells could be established.

We claim:
 1. In a method for decreasing the viability of micro-organismsand/or the activity of enzymes in a contaminated substance by applying ahigh pressure to the substance, the improvement wherein the substance isconducted in a steady flow through a tube, while the pressure differencebetween the entrance end and the exit end of the tube is maintained at100 Mpa or more.
 2. Method according to claim 1 wherein the residencetime of the substance in the tube is at least 1 second.
 3. Methodaccording to claim 1 or claim 2 wherein the substance is a food productor an ingredient for a food product.
 4. Method according to claim 1 orclaim 2 wherein the ratio of the length and the diameter of the tube isat least
 1000. 5. Method according to claim 1 or claim 2 wherein thetube contains at its upstream end a chamber of which the diameter is atleast 5 times greater than the remainder of the tube.
 6. Methodaccording to claim 1 or claim 2 wherein the temperature rise of thesubstance during tube passage is less than 10° C.
 7. Method according toclaim 1 wherein the pressure difference between the entrance end and theexit end of the tube is maintained at 300 Mpa or more.
 8. Methodaccording to claim 2 wherein the residence time is at least 2 minutes.9. Method according to claim 8 wherein the residence time is at least 10minutes.
 10. Method according to claim 4 wherein said ratio is at least10,000.
 11. Method according to claim 5 wherein the chamber is in theform of a tube.
 12. Method according to claim 6 wherein the temperaturerise is less than 5° C.